During the 2016 campaign, Mr Comey twice commented publicly on the FBI's investigation into Hillary Clinton's use of a private email server while secretary of state.

He reopened the case in the crucial final days of the campaign.

But he has defended his decision not to speak about the investigation into Russian interference, which may have involved Mr Trump's campaign staff, because it was early days and "we didn't know what we had".

"It just makes it more painful. You've still got to do the right thing and choose as — again, as I thought about it, between bad and catastrophic. You've got to choose bad everyday over catastrophic.

"The notion that because of the choices we had to make we had an impact, leaves me feeling ill. Now, it may sound odd to say, but that wouldn't change the way I thought about the decisions."